"If your device isn't decrypting in TWRP, chances are you still have the old crypto key format from the 3.5.1 release firmware.
You can solve this by updating to 3.5.3 and changing your boot up password in your device OS's settings.
By changing your password after updating to 3.5.3 or newer, your OS will rewrite your crypto key to be compatible with TWRP.
You are free to disable your password after this if you don't want one, and TWRP will decrypt with the default password."